1. Field of Invention
The present invention relates to a development apparatus which is used in an image forming apparatus, such as a copier, a printer, a facsimile machine, or the like, and visualizes an electrostatic latent image formed on an image carrier using developer, and an image forming apparatus comprising the same. More particularly, the present invention relates to a so-called trickle development apparatus which employs two-component developer containing toner and carrier, and supplies the toner and the carrier while discharging excess developer, and an image forming apparatus comprising the same.
2. Related Art
There is a known development apparatus which comprises a development container which accommodates two-component developer containing toner and carrier, an agitating-transport member which agitates and transports the developer in the development container, and a developer carrier which is provided close to or in contact with an image carrier, and visualizes an electrostatic latent image on the image carrier. In such a development apparatus of the two-component development type, a degradation in development performance is caused by a reduction in carrier performance, such as adhesion of the toner to the carrier (so-called “spent phenomenon”), peeling off of carrier coating, or the like.
As means for solving the above-described problems, for example, techniques as disclosed in JP H07-111598 B (hereinafter referred to as “Patent Document 1”) and JP H02-21591 B (hereinafter referred to as “Patent Document 2”) have been proposed. Patent Documents 1 and 2 disclose a so-called trickle development apparatus. Specifically, in the development apparatus, a small amount of carrier is replenished when replenishing toner, and excess carrier (or developer which is a mixture of carrier and toner) is discharged from a development unit, so that carrier performance is caused to be kept at substantially a constant level in the development unit, thereby stabilizing development performance over time.
As an important operation, for example, such a trickle development apparatus discharges an amount of developer corresponding to the amount of developer (amount of carrier) replenished to the development container, out of the development container, thereby keeping constant the amount of the developer contained in the development container. Here, in the development apparatus described in Patent Documents 1 and 2 discharges developer using a spontaneous discharge method (overflow method) with which the developer is overflowed. Specifically, in a desired state, the bulk of the developer is increased by an amount of developer replenished into the development container, and a portion of the developer located higher than a position of a developer discharge outlet for discharging developer is discharged therethrough. Due to such developer discharge, the amount of the developer in the development container is spontaneously kept constant.
However, a development apparatus which employs a simple overflow method has a drawback such that the height of a developer level in the development container is likely to vary due to external disturbance (e.g., the installed state of the image forming apparatus itself is tilted, an impact is externally applied, etc.), or an operation of agitating the developer in the development container. As a result, the developer is excessively discharged, so that the amount of the developer in the development container is not stable, possibly leading to an adverse influence on development performance of the development apparatus, and further, image formation of the image forming apparatus.
To solve such a problem, there have been various proposals as follows.
For example, a development apparatus has been proposed in which a shutter member is provided on a wall surface of the development container so as to detect a predetermined print amount or the amount of the developer in the development container. Based on a resultant detection signal, the shutter member is opened and closed so as to discharge excess developer.
Also, as disclosed in, for example, JP 2001-265098 A (hereinafter referred to as “Patent Document 3”), a development apparatus has been proposed in which two screw-type agitating members are provided, a developer discharge outlet is formed in a lower portion at a transport direction downstream side of the agitating members, and a member for imparting reverse transport force is provided at a developer discharge outlet side of the agitating member, so that the height of a developer level in the vicinity of the developer discharge outlet is forcedly lowered, whereby, when the developer exceeds a predetermined height of the discharge outlet, the developer is discharged. Although this development apparatus is basically an overflow-type development apparatus, the reverse transport member is provided so as to impart the reverse transport force, thereby preventing a significant change in the height of the developer level in the vicinity of the developer discharge outlet even when the image forming apparatus itself is tilted or an impact is externally applied. As a result, developer discharge can be stably carried out.
However, the above-described conventional development apparatuses have the following problem.
In the trickle development apparatus, the amount of excess developer is considerably small. Therefore, when the development apparatus is provided with a shutter member which is opened and closed so as to discharge excess developer, it is considerably difficult to discharge a considerably small amount of developer with high precision by opening and closing the shutter member. Therefore, in the above-described method, it is considerably difficult to maintain the developer of the development container in a desired amount with high precision.
In the development apparatus disclosed in Patent Document 3, since the developer discharge outlet is provided below the developer level in the development container, developer discharge is significantly affected when a property of the developer in the development apparatus changes. Specifically, when the flowability or bulk density of the developer in the development container is changed due to the environment or a change over time, the flow of the developer is likely to be significantly affected by an operation of the reverse transport member. Therefore, the height of the developer level in the vicinity of the developer discharge outlet may change to a level different from a desired state. When the whole image forming apparatus is tilted in a longitudinal direction of the development apparatus, the reverse transport member effectively works. However, when the whole image forming apparatus is tilted in a direction perpendicular to the longitudinal direction, i.e., a printing direction, the beneficial effect of the reverse transport member is not sufficiently obtained, so that the height of the developer level in the vicinity of the developer discharge outlet may be unstable.